Methods of charging and recharging batteries comprised of rechargeable cells are well known in the prior art. There exist several different varieties of rechargeable cells commonly used, such use depending upon the current draw requirements of the particular devices to be powered. Many of the methods for determining charge termination typically rely on voltage-sensing alone, cutting off or changing the rate of charging when the voltage at the terminals of the battery has reached a predetermined value, often compensating the voltage cutoff values for variations in the ambient temperature. Other exotic techniques, such as employed by Saar, U.S. Pat. No. 4,392,101, examine the second derivative of the battery voltage with respect to time in order to predict internal electrochemical changes within the cell and control the method of charging. Some techniques simply rely on the passage of charging time alone. However, none of the techniques taught in the prior art describe a method of simultaneously monitoring voltage, current, and time to determine the charge condition of a lithium type cell. Further, no technique in the prior art teaches monitoring the direction of current flow to determine whether the cell is sourcing or sinking current, thus charging or discharging energy, to analyze the state of charge of the cell.
Electrochemical cells do not charge linearly over the duration of the charging process. Due to the chemical characteristics of the particular elemental combinations used for storing electrical energy as chemical energy, a cell passes through a series of states of varying electrical behavior as observed from the terminals of the electrochemical cell. In general, electrochemical cells draw higher amounts of current when charging from a state of low potential energy than when charging from a higher state of potential energy when nearly fully charged. The electrochemical cell exhibits variations in voltage as well, correlating to the changing current draw as energy is delivered into the cell.
Current rechargeable battery systems employ either nickel-cadmium or lead acid cells, which perform well in most applications, but these cells have certain disadvantages when it comes to portability. For a given amount of charge capacity, these cells are relatively heavy. Demand for batteries with higher energy densities at lower costs is fueling the search for alternatives to nickel-cadmium and lead-acid cells. Nickel-metal hydride cells have greater energy density than nickel-cadmium cells, resulting in longer operating times, but are also more expensive. Zinc-air cells offer greater energy densities as well, but cannot be charged too rapidly and have shorter operational lifespans. Lithium cells are well suited to applications requiring low current draw for extended periods of time, and are inexpensive. The problem with lithium cells is that the cell may explode if overcharged due to the instability of pure lithium metal.
Because of the volatility of cells manufactured from pure lithium metal, battery manufacturers are developing variations of lithium cells in order to reduce the dangerousness of pure lithium. Lithium-ion cell technology utilizes a metal-oxide alloy at the anode, the positive electrode, and a carbon based structure at the cathode, the negative electrode. The two electrodes are separated by an organic electrolyte wherein lithium-ions flow from one electrode to the other producing the electrochemical reactions for storing or discharging energy. Another variation of the lithium based cell is the lithium polymer cell in which the anode is composed of a metal-oxide alloy and the cathode is composed of a lithium-metal foil. The main difference between the lithium-ion cell and the lithium polymer cell is that the electrolyte is composed of a solid polymer rather than a liquid as in the lithium-ion cell.
Consideration of the charging characteristics of the lithium-ion cell is important for maximizing the amount of energy the cell will hold, and to prevent overcharging. Additionally, faster charging times to peak cell capacity may damage the cell if overcharging occurs. As the cell becomes overcharged, the voltage it is capable of producing will decrease. In general, electrochemical cells also exhibit hysteresis characteristics, or memory effect, such that when a cell is not fully discharged before being recharged, the cell, after being fully charged from the intermediate charge state, will discharge only to the previous charge level, eventually causing the cell to prematurely reach the end of its useful life. The same may occur with overcharged cells because the output voltage of the cell decreases when charged beyond maximum capacity. Thus, the process of recharging lithium cells must account for the electrochemical behavior of the lithium cell.
It is therefore an object of the invention to implement an improved method and apparatus for charging lithium cells.
It is another object of the invention to determine the charge condition of a lithium battery.
It is a further object of the invention to apply the appropriate charging techniques to a lithium battery based upon the determined charge condition.
Another object of the invention is to allow the charging process to accommodate variations in lithium cell fabrication.